This invention relates to a method for manufacturing a bearing boss with a thermal spray coating layer, which can be suitably used, for example, at a pivotal pin joint portion of a working mechanism of a construction machine or the like.
Generally, construction machines such as hydraulic power shovel type excavator machines, for example, are equipped with a working mechanism for ground excavating operations or for performing other ground working jobs. The working mechanisms of this sort generally have a boom, an arm and a bucket which are pivotally connected with each other through a bearing device.
More particularly, in the case of a hydraulic power shovel type excavator, for example, a boss is provided at a fore end of an arm, which is one of two members to be pivotally connected to each other. while a pair of brackets are provided on the part of a bucket or the other one of the two members to be pivotally connected. The boss is fitted between the paired brackets, and pivotally connected to the latter by the use of a joint pin. A bush is fitted in a joint pin hole in the boss, and the joint pin is slidably received and supported in the bush on the inner peripheral side of the boss.
Further, according to a prior art arrangement, end faces of the boss are held in sliding contact with end faces of the brackets thereby to support thrust loads, which are exerted on the bucket during an excavating operation.
In the case of a prior art machine which is arranged to handle a large amount of soil or dirt by the bucket each time, it is inevitable that soil frequently gets into small clearances between the boss and the brackets. Rotations of the bucket in this state invariably result in accelerated abrasive wear of end faces of the boss due to abrasion by dirt.
Regarding other prior art bearing devices of this sort, for instance, there has been known in the art a bearing device as described in Japanese Laid-Open Patent No. H9-184518. In this case, in order to enhance resistance to abrasive wear, a hard material, for example tungsten carbide, is provided on end faces of a boss which are in contact with brackets.
In the case of the prior art just mentioned, a bush is fitted in a joint-pin hole on the inner peripheral side of the boss by press-in fitting for slidably supporting a joint pin therein. In this case, however, high precision machining is required in shaping a joint pin hole in the boss according to the outside diameter of the bush.
In this regard, according to this prior art arrangement, in order to form a joint pin hole of high precision, an original hole on the inner peripheral side of a structural base material for a boss unit is subsequently machined precisely to a specified diameter by a finish machining operation.
However, in a case where a hard thermal spray coating layer is formed on the entire end faces of a boss in the manner as in the bearing device of the second prior art mentioned above, a number of problems arise at the time of finishing a joint pin hole in a subsequent precision machining stage, as described below.
Namely, at the time of machining an original hole of a structural base material for a boss unit into a joint pin hole subsequently to forming thermal spray coating layers on its end faces, part of the thermal spray coating layers need to be removed along with stock of the base material depending on an allowance range in the machining operation. Upon partial removal, cracking, defoliation or other damages however are likely to occur to the hard thermal spray coating layers on the boss. In addition, there arises another problem that the service life of a cutting tool can be shortened to a considerable degree by machining the thermal spray coating layers.
In view of the problems with the prior art devices, it is an object of the present invention to provide a method for manufacturing a bearing boss with a thermal spray coating layer, which can prevent cracking or damages as would normally occur to a thermal spray coating layer on an end face of an annular structural base material of a boss when machining an original hole in the structural base material into a final joint pin hole of a specified diameter, and which can contribute to improve the quality of the thermal spray coating layer.
According to the present invention, in order to achieve the above-stated objective, there is provided a method for manufacturing a bearing boss with a thermal spray coating layer, comprising the steps of: preparing a structural base material for a boss, the structural base material having an annular body and containing an original hole to be ultimately into a joint pin hole of a predetermined diameter; masking an end face of the structural base material on surface areas corresponding to an allowance range of a machining operation; and forming a thermal spray coating layer on unmasked surface areas on the end face of the structural base material by the use of cemented carbide material.
With the arrangements just described, at the time of forming a thermal spray coating layer of a hard material on an end face of a bearing boss, an end face of an annular structural base material for a boss unit is masked as a non-coating surface area in those portions corresponding to an allowance range of a machining operation by which an original hole in the structural base material is ultimately shaped into a joint pin hole of a predetermined diameter. Accordingly, a subsequent machining operation on the original hole in the structural base material does not require to remove part of the hard thermal spray coating layer along with stock of the base material, permitting to shape the original hole into a joint pin hole of a specified diameter in a facilitated manner.
Further, according to the present invention, an inner masking member is fitted in the original hole of the structural base material at the time of masking the end face thereof, and a thermal spray coating layer of a hard material is formed on an annular surface area circumventing outer periphery of the inner masking member.
In this case, an inner masking member is fitted in an original hole in an annular structural base material for a boss unit, and in this state a thermal spray coating layer of a hard material is formed on the end face of the structural base material. Therefore, a thermal spray coating layer is formed on the end face of the structural base material only on an annular coating area which circumvents the inner masking member.
Further, according to the present invention, inner and outer masking members are fitted in or on inner and outer peripheral sides of the structural base material at the time of masking the end face, and a thermal spray coating layer of a hard material is formed on the end face on an annular surface area between the inner and outer masking members.
In this case, an inner masking member is fitted in an original hole in an annular structural material for a boss unit while an outer masking member is fitted on outer peripheral side of the structural base material, and in this state a thermal spray coating layer of a hard material is formed on the end face of the structural base material only on an annular surface area which is exposed between the inner and outer masking members.
Further, according to the present invention, the inner masking member is arranged to have a masking portion with an outside diameter larger than a joint pin hole to be ultimately formed by machining an original hole in the base material.
Alternatively, according to the present invention, the inner masking member is arranged to have a masking portion with an outside diameter larger than an allowance range in a machining operation by which an original hole in the structural base material is ultimately formed into a joint pin hole of a predetermined diameter.
Therefore, in a stage of machining a joint pin hole, the machining operation does not require to remove part of the hard thermal spray coating layer along with stock of the structural base material, and the original hole in the structural base material can be easily machined by the use of a cutting tool.